Bird Networks in the Amazon (complexity)


Relationships between organisms are broad


  • Competition
    • today


  • Mutualisms
    • jigsaw


  • Predation and Consumption
    • next week


  • Parasites and hosts


  • Humans vs other species
    • friday data exercise

What is competition?


Species relationships may have benefits/costs to fitness


WHO: Intra vs Inter-specific competition


Intraspecific competition limits population growth


Competition is a powerful driver of evolution





  • Natural selection favors individuals that do not compete


  • Drives niche separation, specialization and species diversification


  • Directional selection can occur causing character displacement

Gause’s Law: Theory of Competitive Exclusion






  • Two species cannot occupy the same niche if they need the same resource


  • Species cannot coexist at constant populations
    • winner and loser


  • Leads to local extinction of weaker competitor or shift to a different ecological niche

Gause’s test tube Paramecium study


Red vs Grey Squirrel in Britain (Murphy 2015)


Competitive exclusion, however, is rare in nature




  • Resources are variable/limiting in an environment
    • leads to symmetric competition


  • Species compete for multiple resources
    • exclusion based on a single resource is too simple


  • Complete niche overlap (test tube) is rare


  • TAKE HOME: Greater overlap = greater competition

Competition does result in the ‘Realized Niche’


Joseph Connel: Competition among barnacles


Can’t we all just get along?


Co-existence and natural selection


  • Overlapping species evolve to use a niche differently
    • resource partitioning = division of resources


  • Species whose niches only partly overlap coexist easier
    • use the environment differently (niche differentiation)
    • evolve traits to use different/less overlapping resources (character displacement)


  • Co-existing species can overlap by using different forms of resources (niche complementarity)

Co-existence through resource partitioning in birds


Co-existence through resource partitioning in lizards


Niche complementarity: different use of microhabitats


Coexistence and character displacement: